Impossible to delete a User Group

Go to the User Groups item and try to delete one of groups. You'll get an exception. The group should be removed.
In scope of this task, please, take a closer look at Hibernate mapping and introduce corrections if you think they should take place. Make sure that when you remove group, users are not actually removed from DB.
Also, note, that in order to delete objects from DB you should use Session#delete(entity) instead of using bulk delete via HQL.